1. Field of the Invention
The present invention relates to a method of maintaining photolithographic precision alignment for a wafer after it is bonded. The method is performed by etching two cavities on a top wafer at the position corresponding to an alignment marks made on a bottom wafer. Afterwards, the bonded top and bottom wafers are treated with high temperature annealing process and thinning process successively so as to form a membrane structured wafer until the alignment marks made on the bottom wafer is exposed thereby providing a means for aligning the circuit pattern formed on the membrane to the structure of the bottom wafer.
2. Description of the Prior Art
In a book issued by Petersen et. al. in 1988 “Silicon Fusion Bonding for Pressure Sensors.” The book introduced a method of applying the wafer bonding process to fabrication techniques for a pressure sensor. It disclosed that a new membrane structured wafer can be obtained after bonding two wafers by means of the process of fusion bonding. In the book, the technology of photolithography is discussed in detail since the performance of this process will significantly affect the property and size of the device.
The instrumentation usable for exposure alignment in lithography can be generally divided in two categories, i.e. a printer and a stepper. The printer may be a contact type or a proximity type, the stepper is a projection type. The resolution of the former is in the range of several micrometer, on the other hand, that of the latter can be in the degree of sub-micrometer.
For some of the micro sensor devices, it is necessary to carry out the layout of circuit on a membrane structured wafer. The characteristic of the device will be better if the cavity of bottom wafer correspond with the device structure on the membrane. In general, there are two categories for alignment in a membrane structured wafer.
The first one is that the printer process is exclusively adopted in wafer photolithographic process since it is possible for double sides alignment to assure precise arrangement for components. However, it has the inherent disadvantages that the photo resolution is poorer than that of the stepper. The resolution in line width becomes more problematic as the size of the components is minimized. As the number of masks which uses the printer is increased, the alignment between layers loses precision resulting in degrading the performance of the device. The wider line width results in enlarging the device size and extravagating the production cost.
The second one is fabrication of wafer structure before bonding is carried out by the stepper, on the successive procedures after the wafer is bonded are left to the printer. However, during printing, the shortcomings of dissatisfactory alignment precision between layers and a broader line width stated above remain the same.
Meanwhile, for any micro sensor device which is fabricated by means of wafer bonding technique, the two face alignment capability is necessary to compensate the inherent shortcoming of its exposure alignment equipment. For this reason, the successive photolithographic process shall only depend on a printer which has two face alignment capability to perform. Two types of techniques mentioned above which are applicable for the successive fabrication process will be:    1) The wafer bonding technique provided by Kurt Petersen et. al. in which the wet etching is utilized after the thinning process is completed for a bonded wafer so as to fabricate a pressure sensor device after the printer is employed to align the top and the bottom structures of the bonding wafer. As shown in FIG. 1, Nadim I. Naluf presented a method in U.S. Pat. No. 6,038,928 for modification of Kurt Petersen et. al.'s pressure sensor structure. At first, a cavity 12 is formed on a bottom wafer 11 by anisotropic etching process, and then the bottom wafer 11 is bonded with a top wafer 13. Next, the bonded wafer is ground to form a membrane structure. Finally, the backside of the bonded wafer is etched by anisotropic etching process.    2) In U.S. Pat. No. 5,632,854, Andy Nirza et. al. disclosed a method of fabricating a piezoresistive pressure sensor device in which the wet etching is replaced by the dry etching, but a printer is used in aligning a cavity 21 with the structure of device 22 formed on the membrane.It should be pointed out that the above two alignment ways are insufficient to satisfy the high standard requirement for the device to have excellent characteristics with a minimized size. The main reason lies in the fact that the printer which is able to serve for double sides aligning of the wafer has a poor layer to layer alignment precision and reduced line width resolution compared to that of the stepper, on the other hand, the stepper which has better capability for alignment and superior line width resolution is unable to make double side alignment.
In view of this, the present invention provides a newly developed process which can do without the double side alignment capability after the wafer is bonded and grounded, yet the top and the bottom structures of a wafer still can be aligned.